Developmental long-trace profiler using optimally aligned mirror-based pentaprism

A low-budget surface slope-measuring instrument, the developmental long-trace profiler (DLTP), was recently brought into operation at the Advanced Light Source Optical Metrology Laboratory. The instrument is based on a precisely calibrated autocollimator and a movable pentaprism. The capability of the DLTP to achieve submicroradian surface slope metrology has been verified via cross-comparison measurements to other high-performance slope-measuring instruments when measuring the same high-quality test optics. Further improvement of the DLTP is achieved by replacing the existing bulk pentaprism with a specially designed mirror-based pentaprism, which offers the possibility to eliminate systematic errors introduced by inhomogeneity of the optical material and fabrication imperfections of a bulk pentaprism. We provide the details of the mirror-based pentaprism design and describe an original experimental procedure for precision mutual alignment of the mirrors. The algorithm of the alignment procedure and its efficiency are verified with rigorous ray-tracing simulations. Results of measurements of a spherically curved test mirror and a flat test mirror using the original bulk pentaprism are compared to measurements using the new mirror-based pentaprism, demonstrating the improved performance.

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